Method and means for measuring relative velocity



July 8,1958 .J5 `FAZALI-:SKI 2,842,762

METHOD AND MEANS FoE MEASURING RELATIVE VELOCITY 2 Race/'Ver i@ 45k rwentor L/oH/v E ZAL fs/f/ (Ittorneg July 8, 1958 J. F. ZALESKI METHOD AND MEANS FOR MEASURING RELATIVE VELOCITY Filed Sept. 23. 1948 2 Sheets-Sheet 2 Cttomeg pulses,- quencies.

' METHODLND MANS Fon rvinAsURlNG'Y RELATIVE VELOCITY .IolmFiZaleski, Queens Village, NY., assigner toGeneral Precision Laboratory Incorporated, a corporation of NewYork Application septemiier z3, 1948,*seria1'No. 50,861

' aciaims. (ci. 343-48)` This invention'rjelate'sto a system for the measurement of the relative velocity of two bodiesnbyrobservation of the Doppler shift in a microwave frequency,

In pulsed microwave Doppler systems heretofore 'developed :for measurements such as the velocity of van v 'aeroplane 4*relative to the surface of the earth,y theioutput of a coherent oscillator hasbeenv employed'for comparison with the received,- dopplerized signal." By coherent oscillator is meant one `that oscillatesin phase with v the pulse generatorl so that when the received signaly is 'compared .with the output ofthe coherent oscillator the phase relations will be the same as if the received signal were compared directly withlthe pulsewhich generated 'l it. The name coherent oscillator is frequently abbreviated to coho .v In such Doppler systems a stable local microwave genveratorralso has frequently been employed for producing,

by kmixing its output with the received signal, energy having a beat intermediate frequency/which is then ampli- A tied by conventional Vintermediate frequency techniques. I Sucha` microwave generator `'may be, for instance, a.` reflex klystron with automatic frequency control operat-V lfing' at 89.70 mc. to produce, whenmixed with a magnetronpgenerator' frequency of 9000mc., a'beat'frequency :of 30 mc. The name stable local microwave generator is frequently abbreviated to stalo. This name is applied only to microwave generators and not to intermediate frequency generators producing 30 to 60 mc. frequency.

One such Doppler system employingrbothfcoherent1 microwave oscillators andstablelocal microwave oscilla- In' the design ofy a pulsed Doppler system employing Va ',c/oherent' oscillator either thejoscillator must bephased anew by thef'pulsed oscillator at each pulse, andmust maintain its phase coherencepwith great stabilityf'or the interval between pulses, .or else the coherent oscillator must'V force the pulsed generator to 'start each pulse in p hase'withit, lwithout itself being aiected in phase stability. These requirements introduce design corn'-y plexities. i Also, in the kdesign of a pulsed Doppler sysf. tem employing a-stableflocal vmicrowave oscillator to produce an intermediate beat frequency, the oscillator must be frequency stable to such a degree that itsv phase -will not shifty substantially during the interval between a stringent requirement "at microwavef fre- Thep'resent invention eliminates the coherent microwave oscillator andthe stable local microwave oscillator, ij rand substitutes 'for both a single device, namely, a micro- 'wave-resonant cavity. This devicey is Vinherently stable v in frequency and', because it simply' stores andv re-emits microwave .energy and does not generate it, mayv be simple in design and substantial in construction.

The essential purpose of thisninvention, therefore, re-V Y sides .in providing a system wherein'a single ruggedfmech- :gl vtors is described in the copendingapplication, Serial No. 49,926, vtiled September 18, 1948,` of F. B. Berger, etal.

. section 33- for reception of the pulsed*v signals tra'ris'mitted,v

' adjustment. v Y n Theexact nature of therinventionrwill be more Lap-V p parent from the following detailed'descriptionjwhen taken i ce' "Patented July: s, 195s anism is utilized to supplant both a coherent local oscillator (coho) and a stable local microwave generator (stalo) resulting in av system which is at one and thesame; time simpler, more substantial and less likely toget out of with the accompanying drawings, rinlwhich:

Figure l isa schematic diagram of a system incorpo.-

rating `the inventicm.V

cavity. l n b Figure 3 is a schematic diagram of `a modified lsystem.

tion 'of the system.

Referring now'to Fig. ly a pulsed microwave generator l 'Y l1 which may 'conveniently be of the magnetron'type generates pulse signals. of a frequency f which may fall K within theK, X, S or other microwave band suitablerfor the purposes `of measurement based on the Doppler principle. ,i The pulse signals so generated aretransmitted through a wave guide section V31, take-off 12, wave guide section 32 to an antenna of any `desired construction where they are radiated in a suitable direction to permitreection thereof. For example, the apparatus illustrated 'may be mounted on an aeroplane and the signals radiated towards the ca-rths surface in which case kthe reflected signals will differ in frequencyffrom the transmitted j signals by an amount'd namely, the Doppler shiftin frequencies.

Thesel reflected signals new `having a frequency `f|+d are received by the antenna 14 and transmitted through wave `guide 40 tosuitable receiving and mixing means,

Vto be described more fully hereinafter..` n

In order thatlthe transmitted and received signals may be directed only in the proper paths despite thefact that a single antenna 14 is Vused for both transmission; and reception of signals, an ATR tube 43 ips-connected to the Vwave guide section 32 at a suitable distancefrom the juncture ofsections 32 and 40 and a TR-tube: 4.4 is inserted in the wave guide section 40 at a suitable' distance from the same juncture. As is-well Yunderstood inthe radar art, the high energy level .of thepulse sig-" nals generated byV themagnetron/,ll will cause-both'v theVY ATR' tube `4.3 and the 'TR tube'44 togi're and these ,signals will be transmitted onlyto the antenna`14,'being blocked, from aecting-the delicate "receiving apparatus by the iirirxg"`,ofV the TR'tube'll, whichin Veffect shortcircuits the wave guide sectionl 40 at its juncturewith Y thewave guide Vsection 32.,V On the otherfliand received signals are 'of'low' energy level, so-that-ATR tube 4 3, and TR tube 44 are 'not tired and the waver guidef'sec.-

tion 32` is open. circuitedy for Vthese signals-but theyv are Y permitted to pass freely throughthe .waveguidesection 40 to theu receiving apparatus.

A portion of the energy contained in generated by the magnetron 11 is transmitted to the wave guidepsection 33 through the take-0R12, which may conveniently be a simple-irisfin the'waveguide-seetion 31, Y a probe, or loop, or a directional coupler as.` theV occa,-V

sion,V demands. j

A microwave-resonant cavity 18 is` connected, throug'l1-Y the mediuml ofY a-waveguide section 34fto the waveguide iThis microwave-resonant lcavity .mayV be ofthe-forrnfas disclosed4 more in' detail in Fig., 2 wherein it@ is v illus-` tnatedxas` a. hollow metal cylinder 51` .containing a piston-A 52 .Whoserposition is;V longitudinally-adjustableby'rota-'- Figure 2 illustrates .oneform Vofr microwave-resonant Figure 4 is a schematic diagram offa further modifica-V Ythe pulse y signals f tion of the shaft 54 and gearing 53. The 4longitudinal adjustment of the piston 52 varies the length of the internal cavity existing between the face of the piston 52 .and the end S so that the cavity may be adjusted to resonate at any desired frequency. A fitting 56 constitutes an attachment for a wave guide so that microwave `signals may be introduced into and transmitted out of the cavity. Such ingress and egress may be provided by a single aperture or iris, separate irises may be used or coupling may be constituted by probes or loops as is well understood in the art.

The pulse signals generated by the magnetron 11 and impressed on the resonant cavity 18 result in oscillation or ringing of this cavity and the signals occasioned by this oscillation or ringing ar-e transmitted through guide 13 to amplifier 30, These oscillations being at the natural period of the cavity and excited through loose coupling from the magnetron are equal in frequency and in the `same phase as the magnetron oscillations, that is, the cavity is phase-coherent with the magnetron.

Instead of a single cavity as described above, there may be employed a series of cavities which are arranged in cascade, the free resonant oscillations of one exciting the next to begin oscillating and thus prolonging the total time of persistence of any useful free oscillations. On the other hand, the cavities may be arranged in parallel, all being simultaneously excited by 4the magnetron and giving a combined output of energy greater than could 'be given by one alone. Such cavities also may be arranged in series-parallel or parallel series combinations, resulting in longer ringing time with higher output energy levels.

-In order that the signals received and emitted by the microwave-resonant cavity 18 may be directed only in xthe proper paths, an ATR tube 36 is connected to the Wave guide 33 between take-off 12. and microwave-resonant cavity 18 and at a suitable distance from Ithe junction with guide 34, and TR tube 3'7 is inserted in the wave guide 13 at a suitable distance on the opposite side of the same junction. As is well understood in the radar art, the high energy level of the pulse signals generated by the magnetron 11 and taken off into guide 33 through take-off -12 will cause both the ATR tube 36 and the TR tube 37 to fire, and these signals will be transmitted only to the microwave cavity 18, being blocked from passing on to the amplifier 30 by the firing of TR tube 37, which in eect short-circuits the wave guide section 13 just below its juncture with the wave guide section 34. On the other hand, energy emitted by the oscillating microwave-resonant cavity 18 after its excitation by magnetron 11 has ceased is of low level so that ATR tube 36 and TR tube 37 are not fired. Thus the wave guide section 33 is open-circuited at the location of ATR tube 36 for these signals but they are permitted to pass freely through the wave guide section 13 to the amplifier 30.

The use of ATR tube 36 and TR tube 37 is preferred, but their omission will not prevent operation, simply reducing the efficiency of operation.

The signals produced by the free oscillation or ringing of microwave-resonant cavity 18 persist long enough after the termination of the pulse generated by magnetron 11 to bridge the time between that termination and the reception of the reiiected pu-lse by antenna 14, and have both the same frequency and the same phase as the signals of generator 11. The signals resulting from oscillation of the microwave-resonant cavity are transmitted through `wave guide 13 and are preferably amplified by microwave amplifier 30, which may be of any type for the amplification of microwaves, such as a klystron tube or the like. Its inclusion is preferable but is not essential to the operation of the circuit.

The amplified signals of frequency f in wave guide lare impressed on a crystal modulator 22 which is also energized by energy derived from an intermediate generator 19preferably crystal-controlled, generating continuous wave signals of a frequency I. F. say 30 to 60 megacycles. These signals are modulated or mixed by the modulator 22 resulting in signals having frequencies f-l-I. F. and f.-1I. F. These signals are impressed on a filter 38, which passes signals of only one frequency, such as f-I. F. and bars signals of all other frequencies. Filter 38 is preferably of the tunable cavity type such as is customarily used for wavemeter purposes. j

The microwave energy of frequency -I. F. passed by filter 38 is led through wave guide 3S to arm 39 of hybrid junction (magic tee) 41, while aforesaid reected signals transmitted through wave guide 40 Iare at the same time led to the `collinear arm 42 of the same hybrid junction. The frequency of the latter energy is that of the generator `modified by the Doppler shift, or f-i-d.

The Ihybrid junction 41 is depicted in a single plane for simplicity, but arms 39 and 42 are intended to represent collinear arms, arm 45 the shunt Iarm and `arm 47 the series arm. In place of this form of junction any other form having similar properties, such as the rat race form, or even a properly matched tee may be employed. An output which is substantially proportional to the sum of the two inputs is derived from arm 45 and energizes a mixer 46, which may be of the crystal type. The mix- Ver 46 mixes the inputs impressed on arms 42 and 39 and delivers the modulation products thereof, including a sig- -nal of the frequency I. Fri-al, through a guide 16 which may be a coaxial cable to a receiver 21. The fourth arm 47 of hybrid junction 41 is nonreflectively terminated by absorber 48.

Receiver 21 has as its function the development of the Doppler signal, usually of audio frequency. This reeiver may comprise an intermediate frequency amplifier energized by the signal output derived from the mixer 46, an intermediate frequency amplitude limiter section and a mixing network or demodulator fed by the limiter output and also by intermediate frequency oscillator 19. As heretofore described the signals derived from the mixer 46 have a frequency I. F.-j-d while those derived from the oscillator 19 have a frequency I. F.. The mixing and demodulating of these two signals, therefore, results in a signal having the frequency d, namely, the Doppler frequency desired to be obtained. This signal of the Doppler frequency may be impressed on the output terminals 49 for utilization purposes and its frequency may be further indicated on the indicator 29.

Thus the microwave energy generated by the magne- Vtron is radiated, reflected and received but changed in frequency by the amount of the Doppler shift. The rc- `ceived signal is compared with a signal whose frequency is derived from the magnetron but which is momentarily stored in the microwave-resonant cavity, and the difference in frequency of Vthe received and stored signals which constitutes the Doppler shift in frequency, is developed in the receiver. The Vcoherent oscillator function is exercised by the microwave-resonant cavity; the stable local microwave oscillator function is exercised by the same cavity in cooperation with the oscillator 19. Thus this invention having neither coherent oscillator nor stable local microwave oscillator, utilizes a microwaveresonant cavity and I. F. oscillator which discharge the functions of both while at the same time avoiding thc disadvantages normally inherent in the use of such dcvlces.

The schematic drawing of Fig. 3 illustrates another embodiment of the invention. The constancy and frecdom from drift of a cavity oscillator permits the beating of its output directly with theV received dopplerized signal, thus directly creating in a mixer a signal of the Doppler frequency in a single step. This modification therefore, eliminates the generation and use of an intermediate frequency.

InrFig. 3 a magnetron generator 81 generates microwave pulses the energy of which flows in great part through take-off 59 to antenna 61. A portion of the I `transmitted,energyjis diverted'by'a take,oif59which may Abe,fajdirectional. coupler, a simple iris, 'on other energy-r abstracting means, land enters a microwaye-resonantcavity .76 tuned tothe frequency f; emitted by generator 8,1.

The cavity may if desired be aided in operation by TR` and ATR tubes as previously described. The free (ringing) oscillations of the cavity are-transmittedr to one collinear arm 64of hybrid junction 62, which through*` a sec;V

ond collinear arm 66 also receivesthe vdopplerized reected .signal through the switching'action of TRl and ATRv tubes '77'V and 7S., A third series arm 63 of hybrid vjunction V62 contains a nonreectiugtermination and a fourth. or output. shunt arm 67, is energized by the energies -e'ntering collineararms 641v and 66. Thisarm feeds a mixeror demodulator 68, the output of .which conasthe coherentoscillator, and there is no needy for the functions of either a lstable local microwave oscillator or an intermediate' frequency oscillator. Y Y. Infthe modification illustratedin Fig. 4v a system is de-V picted which issimilar tothat of Fig. 3y except. that a freel runningz local oscillator is. usedso that the signals are i' Lestanca 117 so that two signalsyone'having afrequencyof 30 l Y Y mc. and the other mcA-d are amplified thereby, mixed and demodulated producing an output: signal atA the` 108,'-while the signal echoed by the resonant-#cavity 103` having 'a frequency f is transmitted through the collinear arm106 and both-are impressed on the output shunta-rm 113 ofthe hybrid junction 107fandY the mixer or demod-k ulator 114vcarried thereby.v The output of thel mixer 1.14,

therefore, vconstitutes the demodulated product'fof thesetwo signals, namely, aY signal having afrequency of- 30 mc. This signal maybefurther amplied'ifA desired by an amplifier 116 before beingimpressed `on thefinputof the receiver 117'. Y Y

The reiected signals received have undergone a'Doppler shift in frequency and: therefore have a frequency of'f-j-d where d is'the Doppler shift in frequency are impressed through thewave lguide section 99 on the collinear armV 118` of a hybrid junction 119, the other collineararm 121 of which hasimpressed thereon a portion Vof the energy generated. byfthelocal. j' Y i oscillator l109. These signals are transmittedrt'o 'the shunt l arm 1,22 and the mixer yor demodulator 123.car-ried there-V by where their demodulated product is obtained and transmitted thro inputof thereceiver 117. l i Y lInasmuch asthe .Signals impressed on the 'mixer 123 .have frequencies of fi30 mc. and f-i-d, {theidifference frequencysignal obtained thereby willbe ar signal of the' frequency 30 mc.+d:" Y 2 This signaljis impressedlonjthe input of thereceiver terminals 126' and 12'7Hwhich has-a frequency d, thatV .is, theDoppler shift in frequency. VThis signal, then,

amplified by the receiver at intermediate frequency rather than Vaudio frequency resulting somewhat; less noisy operation. This system also partakes of the system de# ,i picted in'Fig; 1 but uses fewer components which is advantageous where v.weight and space are factors to` be considered.

' A magnetron guid'esection 94 to an antenna 96which acts both to radiate. the. transmitted energy Aand',tofreceive the reflected energy..` Inasmuch as this, singl Y antcnnaiperforms both f. Y 'the transmitting and, receiving vfunctions?zgtjduplexing arrangement consisting of 'anATRf tube97 andaTR tube ",798, Aare properly located' with respecttothej transmitting fi Wave guide` section'z9i4. and receiving wavej guide section 9.9 as previously'described.

- .The take-off 93 transmits ajportion'ofv the energy` gen-r .tions' 101 and 102 tothe resonantcavitylti which being excited resonatesH substantially/.at and in phase with Vthe generated'frequencygacting Vasa source lofuo'scillations lvvliiclgrare sustained over. aperiod of time before dying A i j out., 'Thesecontinuingpsignals resultingvfrom excitation Q of 'the`resonant, cavity are transmittedjovoer the wave guidesection-194 to the `collinear arm`106 of the hybridf ljunction 107 the other-.collinear /arm'103 ofwhich has i Virnpressedthereon signals generated'byS'the-freerunningv oscillator 109. Y

If desired, aspreviouslyexplained, thel resonant cavity and the magnetron-may bevconnected together and to the remainder. of the circuit by a duplex'ing circuit consisting of. an ATRA tub'e111 and TR'tube 112.

Assuming that the generated` microwave pulses are ofr a frequencyj, the "local oscillator 109 is'joperated 'at al frequency whidh departs therefrom .bygan amount equal f to` the .desiredintermediate frequency, for example, 30 me The local oscillator'may'generate signals which are above 'or`.. be low the 'frequency ofthe lsignals Ygeneratedby thefmi'crowave generator'91ias`desiredand hence thefrequency-ofthej signals generated by the oscillator 109 are ydesignatedas having" a frequency fifmc; f e

' fdl-30 impressedonzthe collineararm l 91,' or other microwave pulse generator 1 transmitslthel microwave pulseenergyjgenerated therebyY lthrougha wave guidefsecftion 92, take-off9j3gand Waver fated by the magnetron 91 through 'the/wave guide sec.:

may be utilized in any manner. to determineV the relative velocity between-say, an airplane carrying the described equipment and the'surfa'ce of the earth from which-fthe transmitted signals arereectedr As,` in the case of the previously described systems `of Figs'. Vland 3 theseriesarms 128and129l of theghybrfid, vterminated in; anfabs'orptionvr junctions 107 and 119 are load.

o The oscillator 109is not required in this system toghe; t

particularly stabilized 'since variations in the frequencyv of the signal generated thereby are canceled out by the variousdemodulating conversions. YFor example, if the signalV frequency thereof departs .from thel rated value by a frequency Af, the frequency of the signal produced4 bythe demodulator 114 will be 30 meri- Af andthefreY Y quency of the lsignal produced by the demodulator {123 Y .j will be 3,0 mc.j-.A,Jjd. As thesey signals aremixedlandl demodulated by. the receiver andsince 'both containv` the same variation in frequency Af this variation is canceled out and the-resultisstill the Doppler shift frequency d. Y.

What is claimed is: i

1. In a system for determining theV Dopplergshift in frequency produced by the transmission, reflection, and

reception of radiant energy between two relatively moving; Y bodies, a microwave pulse generator on agrst body,

means for radiating the pulse signals Yproducedrby said generator toward a second body for reflectionthereby,r

means 'for receiving said'reflected signals, a microwaveresonant cavlty, a circuit connecting said pulse generator and said resonant cavity whereby said resonant; cavity is'Vr energized by said generated pulse signals resulting in the. production of free oscillationswithin saidl 'cavity whichV endure for a period of time, a mixena circuit inter? connecting said resonant cavity and said mixer for pressing signals on said mixer having a phase and "frequency representative of the phase and frequencyofsaid free oscillations, a ,.circuitfor impressing's'aid:A reflected.y signals on said mixerwherebyj a beat frequencysignal` is obtained,means for 'deriving'an output from'fsaid beat.-

frequency signal which is Vproportionalto the difference in frequency of said radiatedV signals andfsaidgreceived;` signals and switch means in'said circuit connecting .said pulse generator and said resonant cavity forpreventing.

by the antenna '96 .which v ugh the coaxial conductor'dzli to. ,the

the radiation of the free oscillations produced by said cavity.

2. In a system for determining the Doppler shift in frequency produced by the transmission, reflection and reception of radiant energy between two relatively moving bodies, a microwave pulse generator on a first body, means for radiating the pulse signals produced by said generator toward a second body for reflection thereby, means for receiving said reflected signals, a microwaveresonant cavity, a circuit interconnecting said pulse generator and said resonant cavity whereby said cavity is energized by said generated pulse signals resulting in the production of free oscillations Within said cavity which endure for a period of time, a mixer, circuit means for impressing the energy produced by said free oscillations on said mixer, circuit means for impressing said received signals on said mixer whereby a beat frequency signal is obtained whose frequency is equal to the Doppler shift in frequency occasioned by the transmissiomreflection and subsequent reception of said pulse signals switching means in the circuit interconnecting said pulse generator and said resonant cavity for preventing the transmission of signals therethrough during the intervals between the generation of pulse signals by said generator, and said meansfor impressing said free oscillations on said mixer including switching means for preventing the transmission of signal energy therethrough during the period of occurrence of said pulse signals.

3. In a system for determining the Doppler shiftin frequency produced by the transmission, reflection and reception of radiant energy between two relatively moving bodies, a microwave pulse generator on a rst body, means for radiating the pulse signals produced by said generator toward a second body for reilection thereby, means for receiving said rellected signals, a microwaveresonant cavity, a circuit interconnecting said pulse generator and said resonant cavity whereby said cavity is energized by said generated pulse signals resulting in the production of free oscillations within said cavity whichl endure for a period of time, a generator of continuous wave oscillations, a rst mixer having its input connected to said resonant cavity and said continuous wave oscillation generator producing a rst beat frequency signal from said continuous wave oscillations and said free oscillations, a second mixer having its input connected to receive said iirst beat frequency signal and said received signals producing a second beat frequency therefrom, a receiver including a demodulator, means impressing said second beat frequency and said continuous wave oscillations on said receiver whereby the demodulated signal derived therefrom constitutes a signal representative of the Doppler shift in frequency occasioned by the transmission, reflection and subsequent reception of said pulse signals.

4. A system in accordance with claim 3 in which lter means is connected between the output of said rst mixer and the input of said second mixer.

5. A system in accordance with claim 3 in which switching means is incorporated in the circuit interconnecting said pulse generator and said resonant cavity for preventing the transmission of signal energy therethrough during the intervals between the generation of pulse signals and switching means is incorporated in the circuit connecting said resonant cavity and said rst mixer for preventing the transmission of signal energy therethrough during the periods of occurrence of said pulse signals.

6. In a system for determining the Doppler shift in frequency produced by the transmission, reflection and reception of radiant energy between two relatively moving bodies, a'micro'wave pulse generator on a lirst body, means for radiating Vthe pulse signals produced by said generator toward a second body for reflection thereby, means for receiving said reflected signals, a microwaveresonant cavity tuned to the frequency of oscillations of said generated pulse signals and energized thereby whereby free oscillations are produced in said cavity which endure for a period of time at least equal to the interval occurring between the transmission of a pulse signal and the subsequent reception of its reflection, means operative by said free oscillations and said received reflected signal for determining the DopplerV shift in frequency occasioned by the transmission, reilection and subsequent reception of said pulse signals and switch means for preventing the radiation of the free oscillations produced in said cavity.

7. A system for determining the Doppler shift in frequency produced by the transmission, reilection and reception of pulsed radiant energy between two relatively moving bodies comprising, a generator of pulsed microwave energy positioned on one of said bodies, means coupled to the output of said generator for radiating said pulsed energy towards the other of bodies, aV microwave resonant cavity tuned to the frequency of oscillations of said generator connected to the output thereof whereby said radiated energy produces sustained free oscillations of the same frequency in said cavity, means for receiving microwave energy reflected from the other of said bodies, a mixer coupled to said cavity and receiving means producing from the received energy and the free oscillations generated by said cavity a signal whose frequency is representative of the diierence in frequency between that of the transmitted and received energy, and means preventing radiation of the free oscillations generated by said cavity.

8. A system as defined in claim 7 in which additional means is provided for preventing imposition of the pulsed energy generated by said microwave generator on said mixer.

9. A system for determining the Doppler shift in frequency produced by the transmission, reection and reception of radiant energy between two relatively moving bodies comprising, a microwave pulse generator positioned on one of said bodies, means for radiating the pulse signals produced by said generator toward the other of said bodies for reection thereby, means including said radiating means for receiving said reflected signals, a microwave resonant cavity, a circuit interconnecting said pulse generator and said resonant cavity whereby said resonant cavity is energized by said generated pulse signals resulting in the production of free oscillations within said cavity which endure for a period of time, a mixer, a circuit interconnecting said resonant cavity and said mixer for impressing signals on said mixer having a phase and frequency representative of the phase and frequency of said free oscillations, means connected to said last mentioned' circuit for preventing the imposition of said pulse signals on said mixer, a circuit for impressing said reected signals on said mixer whereby a beat frequency signal is obtained, means for deriving an output from said beat frequency signal which is proportional to the difference in frequency of said radiated signals and said received signals, and switch means in said circuit interconnecting said pulse generator and said resonant cavity for preventing the radiation of the free oscillations produced by said cavity.

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